Memory Effects in Pd-Catalysed Allylic Alkylation: Stereochemical Labelling through Isotopic Desymmetrization
摘要:
H-2-Labelled and O-18-labelled cyclopentenyl esters (+/-)-4 and (+/-)-5 are used as probes for memory effects in Pd-catalysed allylic alkylation. H-2-Labelled alkylation product 6 arising from stereospecific Pd-catalysed reaction of (+/-)-4 was analysed by a novel C-13 NMR method involving H-2-isotope shifts and paramagnetic diastereotopic shifts. When catalysts bearing the Trost modular ligand (R,R)-3 were employed, variable memory effects were observed with the slower reacting chirality mismatched (R)-4 substrate-catalyst pairing. The memory effect is dependent on nucleofuge steric bulk and not pK(a). Attack by [LiCH(CO2CH3)(2)] occurs with reversed site selectivity but (R)-4 remains the mismatched substrate. Mismatched ionisation leading to a Pd-pi-allyl in which (R,R)-3 acts as a monophosphine ligand may explain the memory effect.